Image projection system with a polarizing beam splitter

ABSTRACT

An image projection system includes one or more wire grid polarizing beam splitters and one or more transmissive arrays. A polarized light beams from a light source and pre-polarizer is directed towards the transmissive array which modulates the polarization of the polarized light beam by selectively altering the polarization of the polarized light beam to encode image information thereon and creating a modulated beam. The modulated beam is directed towards the wire grid polarizing beam splitter which acts as an analyzer to separate the modulated beam into reflected and transmitted beams. A screen is disposed in one of the reflected or transmitted beams to display the encoded image information. The polarizing beam splitter is oriented at an angle with respect to the modulated beam such that the reflected beam is directed away from the transmissive array. A plurality of transmissive arrays can be used for different colors. One or more polarizing beam splitters can act as both analyzers and combiners.

This application is a continuation-in-part of Ser. No. 09/862,183 nowU.S. Pat. No. 6,447,120, issued Sep. 10, 2002, and filed May 21, 2001,which is a continuation-in-part of Ser. No. 09/363,256 U.S. Pat. No.6,234,634, issued May 22, 2001, and filed Jul. 28, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an image projection systemoperable within the visible spectrum that includes a polarizing beamsplitter that reflects one linear polarization of light and transmitsthe other. More particularly, the present invention relates to such animage projection system with a compact, light-weight beam splitter thatis comprised of a plurality of elongated, reflective elements forinteracting with the electromagnetic waves of the source light togenerally transmit or pass one polarization of light, and reflect theother polarization.

2. Related Art

Polarized light is necessary in certain applications, such as projectionliquid crystal displays (LCD). Such a display is typically comprised ofa light source; optical elements, such as lenses to gather and focus thelight; a polarizer that transmits one polarization of the light to theliquid crystal array; a liquid crystal array for manipulating thepolarization of the light to encode image information thereon; means foraddressing each pixel of the array to either change or retain thepolarization; a second polarizer (called an analyzer) to reject theunwanted light from the selected pixels; and a screen upon which theimage is focused.

It is possible to use a single polarizing beam splitter (PBS) to serveboth as the first polarizer and the second polarizer (analyzer). If theliquid crystal array is reflective, for example a Liquid Crystal OnSilicon (LCOS) light valve, it can reflect the beam that comes from thepolarizer directly back to the polarizer after encoding the image bymodifying the polarization of selected pixels. Such a system wasenvisioned by Takanashi (U.S. Pat. No. 5,239,322). The concept waselaborated by Fritz and Gold (U.S. Pat. No. 5,513,023). These similarapproaches would provide important advantages in optical layout andperformance. Neither, however, has been realized in practice because ofdeficiencies in conventional polarizing beam splitters. Thedisadvantages of using conventional polarizing beam splitters inprojection liquid crystal displays includes images that are not bright,have poor contrast, and have non-uniform color balance or non-uniformintensity (due to non-uniform performance over the light cone). Inaddition, many conventional polarizing beam splitters are short-livedbecause of excessive heating, and are very expensive.

In order for such an image projection system to be commerciallysuccessful, it must deliver images which are significantly better thanthe images provided by conventional cathode ray tube (CRT) televisiondisplays because it is likely that such a system will be more expensivethan conventional CRT technology. Therefore, the image projection systemmust provide (1) bright images with the appropriate colors or colorbalance; (2) have good image contrast; and (3) be as inexpensive aspossible. An improved polarizing beam splitter (PBS) is an importantpart of achieving this goal because the PBS is a limiting componentwhich determines the potential performance of the display system.

The PBS characteristics which significantly affect the displayperformance are (1) the angular aperture, or the f-number, at which thepolarizer can function; (2) the absorption, or energy losses, associatedwith the use of the PBS; and (3) the durability of the PBS. In optics,the angular aperture or f-number describes the angle of the light conewhich the PBS can use and maintain the desired performance level. Largercones, or smaller f-numbers, are desired because the larger cones allowfor more light to be gathered from the light source, which leads togreater energy efficiency and more compact systems.

The absorption and energy losses associated with the use of the PBSobviously affect the brightness of the system since the more light lostin the optics, the less light remains which can be projected to the viewscreen. In addition, the amount of light energy which is absorbed by thepolarizer will affect its durability, especially in such imageprojection systems in which the light passing through the optical systemis very intense, on the order of watts per square centimeter. Light thisintense can easily damage common polarizers, such as Polaroid sheets. Infact, the issue of durability limits the polarizers which can be used inthese applications.

Durability is also important because the smaller and lighter theprojection system can be made, the less expensive and more desirable isthe product. To accomplish this goal, however, the light intensity mustbe raised even higher, further stressing the PBS, and shortening itsuseful life.

A problematic disadvantage of conventional PBS devices is poorconversion efficiency, which is the primary critical performance factorin displays. Conversion efficiency is a measure describing how much ofthe electrical power required by the light source is translated intolight intensity power on the screen or panel that is observed by peopleviewing it. It is expressed as the ratio of total light power on thescreen divided by the electrical power required by the light source. Theconventional units are lumens per watt. A high ratio is desirable for anumber of reasons. For example, a low conversion efficiency will requirea brighter light source, with its accompanying larger power supply,excess heat, larger enclosures and cabinet, etc. In addition, all ofthese consequences of low conversion efficiency raise the cost of theprojection system.

A fundamental cause of low conversion efficiency is poor opticalefficiency, which is directly related to the f-number of the opticalsystem. A system which has an f-number which is half the f-number of anotherwise equivalent system has the potential to be four times asefficient in gathering light from the light source. Therefore, it isdesirable to provide an improved polarizing beam splitter (PBS) whichallows more efficient harvesting of light energy by offering asignificantly smaller potential f-number (larger angular aperture), andtherefore increases the conversion efficiency, as measured inlumens/watt.

There are several reasons for the poor performance of conventionalpolarizing beam splitters with respect to conversion efficiency whenthey are used as beam splitters in projection systems. First, currentbeam splitters work poorly if the light does not strike them at acertain angle (or at least, within a narrow cone of angles about thisprincipal angle of incidence). Deviation of the principal ray from thisangle causes each type of polarizing beam splitter to degrade theintensity, the purity of polarization, and/or the color balance. Thisapplies to the beam coming from the light source as well as to the beamreflected from the liquid crystal array. This principal angle dependsupon the design and construction of the PBS as well as the physics ofthe polarization mechanism employed in these various beam splitters.Currently available polarizing beam splitters are not capable ofoperating efficiently at angles far from their principal polarizingangles in the visible portion of the electromagnetic spectrum. Thisrestriction makes it impossible to implement certain promising opticallayouts and commercially promising display designs.

Even if the principal ray strikes the polarizer at the best angle forseparating the two polarizations, the other rays cannot diverge far fromthis angle or their visual qualities will be degraded. This is a seriousdeficiency in a display apparatus because the light striking thepolarizer must be strongly convergent or divergent to make efficient useof the light emitted by typical light sources. This is usually expressedas the f-number of the optical system. For a single lens, the f-numberis the ratio of the aperture to the focal length. For optical elementsin general, the F-number is defined as

F/#=1/(2n sin θ)

where n is the refractive index of the space within which the opticalelement is located, and θ is the half cone angle. The smaller theF-number, the greater the radiant flux, Φ_(c), collected by the lens,and the more efficient the device will be for displaying a bright image.The radiant flux increases as the inverse square of the F/#. In anoptical train, the optical element with the largest F/# will be thelimiting factor in its optical efficiency. For displays usingtraditional polarizers, the limiting element is nearly always thepolarizer, and thus the PBS limits the conversion efficiency. It wouldclearly be beneficial to develop a type of PBS that has a smaller F/#than any that are currently available.

Because traditional polarizers with small F/#s have not been available,designers typically have addressed the issue of conversion efficiency byspecifying a smaller, brighter light source. Such sources, typically arclamps, are available, but they require expensive power supplies that areheavy, bulky, and need constant cooling while in operation. Cooling fanscause unwanted noise and vibration. These features are detrimental tothe utility of projectors and similar displays. Again, a PBS with asmall F/# would enable efficient gathering of light from low-power,quiet, conventional light sources.

Another key disadvantage of conventional polarizing beam splitters is alow extinction, which results in poor contrast in the image. Extinctionis the ratio of the light transmitted through the polarizer of thedesired polarization to the light rejected of the undesiredpolarization. In an efficient display, this ratio must be maintained ata minimum value over the entire cone of light passing through the PBS.Therefore, it is desirable to provide a polarizing beam splitter whichhas a high extinction ratio resulting in a high contrast image.

A third disadvantage of conventional polarizing beam splitters is anon-uniform response over the visible spectrum, or poor color fidelity.The result is poor color balance which leads to further inefficiency inthe projection display system as some light from the bright colors mustbe thrown away to accommodate the weaknesses in the polarizing beamsplitter. Therefore, it is desirable to provide an improved polarizingbeam splitter that has a uniform response over the visible spectrum, (orgood color fidelity) giving an image with good color balance with betterefficiency. The beam splitter must be achromatic rather than distort theprojected color, and it must not allow crosstalk between thepolarizations because this degrades image acuity and contrast. Thesecharacteristics must apply over all portions of the polarizer and overall angles of light incidence occurring at the polarizer. The termspathic has been coined (R. C. Jones, Jour. Optical Soc. Amer. 39, 1058,1949) to describe a polarizer that conserves cross-sectional area, solidangle, and the relative intensity distribution of wavelengths in thepolarized beam. A PBS that serves as both a polarizer and analyzer mustbe spathic for both transmission and reflection, even in light beams oflarge angular aperture.

A fourth disadvantage of conventional polarizing beam splitters is poordurability. Many conventional polarizing beam splitters are subject todeterioration caused by excessive heating and photochemical reactions.Therefore, it is desirable to provide an improved polarizing beamsplitter that can withstand an intense photon flux for thousands ofhours without showing signs of deterioration. In addition, it isdesirable to provide a polarizing beam splitter that is amenable toeconomical large scale fabrication.

The need to meet these, and other, criteria has resulted in only a fewtypes of polarizers finding actual use in a projection system. Manyattempts have been made to incorporate both wide angular aperture andhigh fidelity polarization into the same beam splitting device. Therelative success of these efforts is described below. Thin filminterference filters are the type of polarizer cited most frequently inefforts to make a polarizing beam splitter that is also used as ananalyzer. MacNeille was the first to describe such a polarizer that waseffective over a wide spectral range (U.S. Pat. No. 2,403,731). It iscomposed of thin-film multi-layers set diagonally to the incident light,typically within a glass cube, so it is bulky and heavy compared to asheet polarizer. What is more, it must be designed for a single angle ofincidence, usually 45 degrees, and its performance is poor if light isincident at angles different from this by even as little as 2 degrees.Others have improved on the design (e.g. J. Mouchart, J. Begel, and E.Duda, Applied Optics 28, 2847-2853, 1989; and L. Li and J. A.Dobrowolski, Applied Optics 13, 2221-2225, 1996). All of them found itnecessary to seriously reduce the wavelength range if the angularaperture is to be increased. This can be done in certain designs (U.S.Pat. Nos. 5,658,060 and 5,798,819) in which the optical design dividesthe light into appropriate color bands before it arrives at thepolarizing beam splitter. In this way, it is possible to reduce thespectral bandwidth demands on the beam splitter and expand its angularaperture, but the additional components and complexity add significantcost, bulk, and weight to the system.

Even so, these improved beam splitter cubes are appearing on the market,and are currently available from well known vendors such as Balzers andOCLI. They typically offer an F/# of f/2.5-f/2.8, which is a significantimprovement over what was available 2 years ago, but is still far fromthe range of F/1.2-F/2.0 which is certainly within reach of the otherkey components in optical projection systems. Reaching these f-numbershas the potential to improve system efficiency by as much as a factor of4. They would also enable the projection display engineer to makepreviously impossible design trade-offs to achieve other goals, such asreduced physical size and weight, lower cost, etc.

In a technology far from visible optics, namely radar, wire grids havebeen used successfully to polarize long wavelength radar waves. Thesewire grid polarizers have also been used as reflectors. They are alsowell known as optical components in the infrared (IR), where they areused principally as transmissive polarizer elements.

Although it has not been demonstrated, some have postulated possible useof a wire grid polarizer in display applications in the visible portionof the spectrum. For example, Grinberg (U.S. Pat. No. 4,688,897)suggested that a wire grid polarizer serve as both a reflector and anelectrode (but not simultaneously as an analyzer) for a liquid crystaldisplay.

Others have posed the possible use of a wire grid polarizer in place ofa dichroic polarizer to improve the efficiency of virtual image displays(see U.S. Pat. No. 5,383,053). The need for contrast or extinction inthe grid polarizer, however, is explicitly dismissed, and the grid isbasically used as a polarization sensitive beam steering device. It doesnot serve the purpose of either an analyzer, or a polarizer, in the U.S.Pat. No. 5,383,053. It is also clear from the text that a broadbandpolarizing cube beam splitter would have served the purpose as well, ifone had been available. This technology, however, is dismissed as beingtoo restricted in acceptance angle to even be functional, as well asprohibitively expensive.

Another patent (U.S. Pat. No. 4,679,910) describes the use of a gridpolarizer in an imaging system designed for the testing of IR camerasand other IR instruments. In this case, the application requires a beamsplitter for the long wavelength infra-red, in which case a gridpolarizer is the only practical solution. The patent does not suggestutility for the visible range or even mention the need for a largeangular aperture. Neither does it address the need for efficientconversion of light into a viewable image, nor the need for broadbandperformance.

Other patents also exist for wire-grid polarizers in the infraredportion of the spectrum (U.S. Pat. Nos. 4,514,479, 4,743,093; and5,177,635, for example). Except for the exceptions just cited, theemphasis is solely on the transmission performance of the polarizer inthe IR spectrum.

These references demonstrate that it has been known for many years thatwire-grid arrays can function generally as polarizers. Nevertheless,they apparently have not been proposed and developed for imageprojection systems. One possible reason that wire grid polarizers havenot been applied in the visible spectrum is the difficulty ofmanufacture. U.S. Pat. No. 4,514,479 teaches a method of holographicexposure of photoresist and subsequent etching in an ion mill to make awire grid polarizer for the near infrared region; in U.S. Pat. No.5,122,907, small, elongated ellipsoids of metal are embedded in atransparent matrix that is subsequently stretched to align their longaxes of the metal ellipsoids to some degree. Although the transmittedbeam is polarized, the device does not reflect well. Furthermore, theellipsoid particles have not been made small enough to be useful in thevisible part of the electromagnetic spectrum. Accordingly, practicalapplications have been generally limited to the longer wavelengths ofthe IR spectrum.

Another prior art polarizer achieves much finer lines by grazing angleevaporative deposition (U.S. Pat. No. 4,456,515). Unfortunately, thelines have such small cross sections that the interaction with thevisible light is weak, and so the optical efficiency is too poor for usein the production of images. As in several of these prior art efforts,this device has wires with shapes and spacings that are largely random.Such randomness degrades performance because regions of closely spacedelements do not transmit well, and regions of widely spaced elementshave poor reflectance. The resulting degree of polarization (extinction)is less than maximal if either or both of these effects occur, as theysurely must if placement has some randomness to it.

For perfect (and near perfect) regularity, the mathematics developed forgratings apply well. Conversely, for random wires (even if they all havethe same orientation) the theory of scattering provides the bestdescription. Scattering from a single cylindrical wire has beendescribed (H. C. Van de Hulst, Light Scattering by Small Particles,Dover, 1981). The current random-wire grids have wires embeddedthroughout the substrate. Not only are the positions of the wiressomewhat random, but the diameters are as well. It is clear that thephases of the scattered rays will be random, so the reflection will notbe strictly specular and the transmission will not retain high spatialor image fidelity. Such degradation of the light beam would prevent itsuse for transfer of well resolved, high-information density images.

Nothing in the prior art indicates or suggests that an ordered array ofwires can or should be made to operate over the entire visible range asa spathic PBS, at least at the angles required when it serves both as apolarizer and analyzer. Indeed, the difficulty of making the narrow,tall, evenly spaced wires that are required for such operation has beengenerously noted (see Zeitner, et. al. Applied Optics, 38, 11 pp.2177-2181 (1999), and Schnabel, et. al., Optical Engineering 38,2 pp.220-226 (1999)). Therefore, it is not surprising that the prior art forimage projection similarly makes no suggestion for use of a spathic PBSas part of a display device.

Tamada and Matsumoto (U.S. Pat. No. 5,748,368) disclose a wire gridpolarizer that operates in both the infrared and a portion of thevisible spectrum; however, it is based on the concept that large, widelyspaced wires will create resonance and polarization at an unexpectedlylong wavelength in the visible. Unfortunately, this device works wellonly over a narrow band of visible wavelengths, and not over the entirevisible spectrum. It is therefore not suitable for use in producingimages in full color. Accordingly, such a device is not practical forimage display because a polarizer must be substantially achromatic foran image projection system.

Another reason wire grid polarizers have been overlooked is the commonand long standing belief that the performance of a typical wire gridpolarizer becomes degraded as the light beam's angle of incidencebecomes large (G. R. Bird and M. Parrish, Jr., A The Wire Grid as aNear-Infrared Polarizer,@J. Opt. Soc. Am., 50, pp. 886-891, (1960); theHandbook of Optics, Michael Bass, Volume II, p. 3-34, McGraw-Hill(1995)). There are no reports of designs that operate well for anglesbeyond 35 degrees incidence in the visible portion of the spectrum. Norhas anyone identified the important design factors that cause thislimitation of incidence angle. This perceived design limitation becomeseven greater when one realizes that a successful beam splitter willrequire suitable performance in both transmission and reflectionsimultaneously.

This important point deserves emphasis. The extant literature and patenthistory for wire grid polarizers in the IR and the visible spectra hasalmost entirely focused on their use as transmission polarizers, and noton reflective properties. Wire grid polarizers have been attempted andreported in the technical literature for decades, and have becomeincreasingly common since the 1960s. Despite the extensive work done inthis field, there is very little, if any, detailed discussion of theproduction and use of wire grid polarizers as reflective polarizers, andnothing in the literature concerning their use as both transmissive andreflective polarizers simultaneously, as would be necessary in a spathicpolarizing beam splitter for use in imaging devices. From the lack ofdiscussion in the literature, a reasonable investigator would concludethat any potential use of wire grid polarizers as broadband visible beamsplitters is not apparent, or that it was commonly understood by thetechnical community that their use in such an application was notpractical.

Because the conventional polarizers described above were the only onesavailable, it was impossible for Takanashi (U.S. Pat. No. 5,239,322) toreduce his projection device to practice with anything but the mostmeager results. No polarizer was available which supplied theperformance required for the Takanashi invention, namely, achromaticityover the visible part of the spectrum, wide angular acceptance, lowlosses in transmission and reflection of the desired lightpolarizations, and good extinction ratio.

There are several important features of an image display system whichrequire specialized performance of transmission and reflectionproperties. For a projector, the product of p-polarization transmissionand s-polarization reflection (R_(S)T_(P)) must be large if source lightis to be efficiently placed on the screen. On the other hand, for theresolution and contrast needed to achieve high information density onthe screen, it is important that the converse product (R_(P)T_(S)) bevery small (i.e. the transmission of s-polarized light multiplied by thereflection of p-polarized light must be small).

Another important feature is a wide acceptance angle. The acceptanceangle must be large if light gathering from the source, and hence theconversion efficiency, is maximized. It is desirable that cones of light(either diverging or converging) with half-angles greater than 20 beaccepted.

An important consequence of the ability to accept larger light cones andwork well at large angles is that the optical design of the imagingsystem is no longer restricted. Conventional light sources can be thenbe used, bringing their advantages of low cost, cool operation, smallsize, and low weight. A wide range of angles makes it possible for thedesigner to position the other optical elements in favorable positionsto improve the size and operation of the display.

Another important feature is size and weight. The conventionaltechnology requires the use of a glass cube. This cube imposes certainrequirements and penalties on the system. The requirements imposedinclude the need to deal with thermal loading of this large piece ofglass and the need for high quality materials without stressbirefringence, etc., which impose additional cost. In addition, theextra weight and bulk of the cube itself poses difficulties. Thus, it isdesirable that the beam splitter not occupy much volume and does notweigh very much.

Another important feature is robustness. Modern light sources generatevery high thermal gradients in the polarizer immediately after the lightis switched on. At best, this can induce thermal birefringence whichcauses cross talk between polarizations. What is more, the long durationof exposure to intense light causes some materials to change properties(typically yellowing from photo-oxidation). Thus, it is desirable forthe beam splitter to withstand high temperatures as well as long periodsof intense radiation from light sources.

Still another important feature is uniform extinction (or contrast)performance of the beam splitter over the incident cone of light. AMcNeille-type thin film stack polarizer produces polarized light due tothe difference in reflectivity of S-polarized light as opposed toP-polarized light. Since the definition of S and P polarization dependson the plane of incidence of the light ray, which changes orientationwithin the cone of light incident on the polarizer, a McNeille-typepolarizer does not work equally well over the entire cone. This weaknessin McNeille-type polarizers is well known. It must be addressed inprojection system design by restricting the angular size of the cone oflight, and by compensation elsewhere in the optical system through theuse of additional optical components. This fundamental weakness ofMcNeille prisms raises the costs and complexities of current projectionsystems, and limits system performance through restrictions on thef-number or optical efficiency of the beam splitter.

Other important features include ease of alignment. Production costs andmaintenance are both directly affected by assembly criteria. These costscan be significantly reduced with components which do not require lowtolerance alignments.

Therefore, it would be advantageous to develop an image projectionsystem capable of providing bright images and good image contrast, andwhich is inexpensive. It would also be advantageous to develop an imageprojection system with a polarizing beam splitter capable of utilizingdivergent light (or having a smaller F/#), capable of efficient use oflight energy or with high conversion efficiency, and which is durable.It would also be advantageous to develop an image projection system witha polarizing beam splitter having a high extinction ratio, uniformresponse over the visible spectrum, good color fidelity, that isspathic, robust and capable of resisting thermal gradients. It wouldalso be advantageous to develop an image projection system with apolarizing beam splitter capable of being positioned at substantiallyany incidence angle so that significant design constraints are notimposed on the image projection system, but substantial designflexibility is permitted. It would also be advantageous to develop animage projection system with a polarizing beam splitter whichefficiently transmits p-polarized light and reflects s-polarized lightacross all angles in the entire cone of incident light. It would also beadvantageous to develop an image projection system with a polarizingbeam splitter which is light-weight and compact. It would also beadvantageous to develop an image projection system with a polarizingbeam splitter which is easy to align. Combining all of these features ina single projection device would offer a significant advance within thestate of the art.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop an imageprojection system which provides bright images with good image contrast,and which is inexpensive. In addition, it has been recognized that itwould be advantageous to develop an image projection system with apolarizing beam splitter which utilizes divergent light (or has asmaller F/#), efficiently uses light energy, has good conversionefficiency, and is durable. It also has been recognized that it would beadvantageous to develop an image projection system with a polarizingbeam splitter with a high extinction ratio, a uniform response over thevisible spectrum, good color fidelity, which is spathic, robust andresists thermal gradients. In addition, it has been recognized that itwould be advantageous to develop an image projection system with apolarizing beam splitter capable of selectively directing either or bothof the transmitted and reflected polarized beams at substantially anyangle. It also has been recognized that it would be advantageous todevelop an image projection system with a polarizing beam splitter whichfunctions adequately while positioned at substantially any incidentangle. In addition, it has been recognized that it would be advantageousto develop an image projection system with a polarizing beam splitterwhich efficiently transmits p-polarized light and reflects s-polarizedlight over all angles within the cone of light, but can also function totransmit s-polarized light and reflect p-polarized light similarly. Italso has been recognized that it would be advantageous to develop animage projection system with a polarizing beam splitter which islight-weight, compact, robust, and easy to align. It is a further objectof the present invention to provide a polarizing beam splitter for usein image projection systems.

The invention provides an image projection system with one or more wiregrid polarizing beam splitters and one or more transmissive arrays. Thesystem includes a light source to produce a visible light beam directedto a first polarizer to polarize the light beam into a polarized lightbeam. The polarized light beam is directed towards the transmissivearray which modulates the polarization of the polarized light beam byselectively altering the polarization of the polarized light beam toencode image information thereon, thus creating a modulated beam. Themodulated beam is directed towards the polarizing beam splitter.

The polarizing beam splitter includes a generally parallel arrangementof thin, elongated elements, supported by a transparent substrate. Thearrangement is configured and the elements are sized to interact withelectromagnetic waves of the modulated beam to generally 1) transmitlight through the elements which has a polarization orientedperpendicular to a plane that includes at least one of the elements andthe direction of the incident light beam, defining a transmitted beam,and 2) reflect light from the elements which has a polarization orientedparallel with the plane that includes at least one of the elements andthe direction of the incident light beam, defining a reflected beam.

The polarizing beam splitter can be oriented at an angle with respect tothe modulated beam such that the modulated beam strikes the firstsurface of the transparent substrate at an angle, and so that thereflected beam is directed away from the transmissive array. Either thereflected or transmitted beam is directed towards a screen fordisplaying the encoded image information.

In accordance with a more detailed aspect of the present invention, thesystem includes a plurality of transmissive arrays for different colors,and one or more polarizing beam splitters acting as both analyzers andcombiners. For example, three transmissive arrays can be used for threeseparate colors. A bandwidth separator can separate the visible lightbeam into a plurality of colored beams each characterized by a differentcolor with different bandwidths. The separator and one or more firstpolarizers together can produce a plurality of colored polarized beams.Each of the transmissive arrays modulate the polarization of therespective colored polarized beam by selectively altering thepolarization of the colored polarized beam to encode image informationthereon, and thus creating a colored modulated beam.

At least two polarizing beam splitters can be used as both analyzers andcombiners. A first polarizing beam splitter is positioned in twodifferent colored modulated beams to combine the two different coloredmodulated beams into a first combined beam. A second polarizing beamsplitter is positioned in the first combined beam and the other coloredmodulated beam to combine the colored modulated beam from the anothertransmissive array and the first combined beam into a second combinedbeam.

In accordance with another more detailed aspect of the presentinvention, the first polarizer also can include a wire grid polarizer.

In accordance with another more detailed aspect of the presentinvention, the image projection system can be realized with at least onepolarizing beam splitter in the case in which only two transmissivepanels are used. In such case, one panel can produce a beam of lightwhich consists of two colors in such a way as to project an image.

Additional features and advantages of the invention will be apparentfrom the detailed description which follows, taken in conjunction withthe accompanying drawings, which together illustrate, by way of example,features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic view of an image projection system in accordancewith an embodiment of the present invention;

FIG. 1b is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 2a is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 2b is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 2c is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 3a is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 3b is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 4a is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 4b is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 4c is a schematic view of another image projection system inaccordance with an embodiment of the present invention;

FIG. 5a is a graphical plot showing the relationship between wavelengthand transmittance for both S and P polarizations of a preferredembodiment of the wire grid polarizing beam splitter of the presentinvention;

FIG. 5b is a graphical plot showing the relationship between wavelengthand reflectance for both S and P polarizations of a preferred embodimentof the wire grid polarizing beam splitter of the present invention;

FIG. 5c is a graphical plot showing the relationship between wavelength,efficiency and transmission extinction of a preferred embodiment of thewire grid polarizing beam splitter of the present invention;

FIG. 6 is a graphical plot showing the performance of the preferredembodiment of the wire grid polarizing beam splitter of the presentinvention as a function of the incident angle;

FIG. 7a is a graphical plot showing the theoretical throughputperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention;

FIG. 7b is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention;

FIG. 7c is a graphical plot showing the theoretical extinctionperformance of an alternative embodiment of the wire grid polarizingbeam splitter of the present invention;

FIG. 8 is a perspective view of the wire grid polarizing beam splitterof the present invention; and

FIG. 9 is a cross sectional side view of the wire grid polarizing beamsplitter of the present invention.

DETAILED DESCRIPTION

Reference will now be made to the exemplary embodiments illustrated inthe drawings, and specific language will be used herein to describe thesame. It will nevertheless be understood that no limitation of the scopeof the invention is thereby intended. Alterations and furthermodifications of the inventive features illustrated herein, andadditional applications of the principles of the inventions asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

As illustrated in FIG. 1a, a display optical train of an imageprojection system of the present invention, indicated generally at 10,is shown. The image projection system 10 advantageously has one or morewire grid polarizers as a beam splitter, indicated at 14 a-c, and one ormore transmissive arrays, 16 a-c. While image projection systems withreflective arrays have been described in the inventor's prior patents,the present image projection system is configured to utilizetransmissive arrays. The wire grid polarizing beam splitter 14 (WGP-PBS)acts as an analyzer to efficiently reflect or transmit light of onepolarization from the transmissive array 16 to a display screen 18. TheWPG-PBS 14 also can advantageously be used as both the analyzer and theimage combiner, as discussed more fully below.

As shown in FIG. 1a, a plurality of WGP-PBSs and a plurality oftransmissive arrays can be used in the system 10, and can eachcorrespond to a different color. For example, three WGP-PBSs 14 a-c andthree transmissive arrays 16 a-c can be used, and can correspond tothree separate and different colors, such as blue, green and red. Aseparator, indicated generally at 20, can be used to separate a visiblelight beam of white light from a visible light source 22 into thedesired colors. Such separators are well known in the art, and caninclude dichroic mirrors 24 disposed in the visible light beam andarranged to separate the white light into colored light beams, each witha different wavelength or bandwidth. It will be appreciated that otheroptical components can be used, as known in the art.

In addition, one or more pre-polarizers or first polarizers 26 a-c canbe disposed in the colored light beams to polarize the colored lightbeams to colored polarized light beams. Alternatively, it will beappreciated that the pre-polarizer or first polarizer can be disposed inthe visible light beam to produce a polarized light beam, while theseparator can be disposed in the polarized light beam to separate toseparating the polarized light beam into the colored polarized lightbeams. Either way, the separator 20 and the first polarizers 26 a-ctogether produce the colored polarized light beams. The visible lightbeam can be separated into various colored light beams first, and thenpolarized, as shown, or the visible light beam can be polarized first,and then separated into various colored light beams. The pre-polarizersor first polarizers 26 a-c can be any type of polarizer. In one aspect,the pre-polarizers or first polarizers 26 a-c can include WGP-PBS, asdiscussed more fully below.

The transmissive arrays 16 a-c are located in the colored polarizedbeams. The transmissive arrays can be transmissive liquid crystalarrays, as are well known in the art. For example, the transmissivearrays can include a liquid crystal layer sandwiched between twoconductive layers. The transmissive arrays 16 a-c modulate thepolarization of the respective colored polarized beams by selectivelyaltering the polarization of the colored polarized beam. Modulating orselectively altering the polarization of the beam encodes imageinformation on the beam, and created a modulated, or colored modulatedbeam. For example, the transmissive arrays 16 a-c can include aplurality of cells or pixels, each independently operable to selectivelyrotate, or unalter, the polarization of the light as it passes throughthe cell or pixel. The liquid crystal material in the transmissivearrays can normally rotate the polarization of the light passing throughthe cell or pixel unless an electric field is applied across the cell orpixel to alter the liquid crystal and to allow the light passingtherethrough unaltered. Alternatively, the liquid crystal material inthe tranxmissive arrays can normally unalter the polarization of thelight passing through the cell or pixel unless an electric field isapplied across the cell or pixel to alter the liquid crystal to rotatethe polarization of the light passing therethrough. The electric fieldcan be applied by the conductive layers on both sides of the liquidcrystal material. A controller can be electrically coupled to thetransmissive arrays to control the modulation of the light, as is knownin the art.

As stated above, the WGP-PBSs 14 a-c are disposed in the respectivecolored modulated beams, and act as analyzers to separate the encodedimage information from the modulated beams. Each WGP-PBS separates themodulated beam into a transmitted beam with one polarization, and areflected beam of another polarization. Thus, the image informationencoded on the modulated beam is separated out from the modulated beaminto either the reflected and/or the transmitted beam. The WGP-PBSs 14a-c can be oriented at an acute or obtuse angle (as opposed to a rightangle) with respect to the respective modulated beam so that themodulated beam strikes the WGP-PBS at an acute or obtuse angle. Thus,the reflected beam from the WGP-PBS is directed away from thetransmissive array. It will be appreciated that orienting the WGP-PBS ata right angle with respect to the modulated beam would result in thereflected beam being directed back into the transmissive array,potentially resulting in noise.

A combiner or an image combiner 27 can be disposed in the reflectedand/or transmitted beams from the WGP-PBSs 14 a-c to combine the imageinformation from the various colored beam into a single image beamdirected towards the screen 18. It will be appreciated that eachreflected or transmitted beam from the WGP-PBS will include the desiredimage in the respective color. The combiner can be of any type, and isknown in the art.

The optical train of the image projection system can be configured withboth an image combiner, and a WGP-PBS. Referring to FIG. 2b, anexemplary image projection system 10 b is shown configured with both acombiner 27 and a WGP-PBS 29. The WGP-PBS 29 is configured as both ananalyzer and a combiner, or to both 1) separate the encoded imageinformation, and 2) combine the various different colored beams. Theimage projection system 10 b is similar in many respects to thosedescribed herein.

The combiner 27 can be disposed in the reflected and/or transmittedbeams from the WGP-PBSs 14 a and b to combine the image information fromthe various colored beams into a first combined beam. The polarizingbeam splitter or WGP-PBS 29 can be disposed in the first combined beamfrom the combiner 27, and another colored modulated beam from anothertransmissive array 16 c. The polarizing beam splitter or WGP-PBS 29 canboth 1) separate the encoded image information from the other modulatedbeam, and 2) combing the reflected and/or transmitted beam from theother transmissive array with the first combined beam from the combiner27 into a second combined beam with the desired image information forthree of the colored beams. Alternatively, it will be appreciated thatthe positions of the combiner 27 and the WGP-PBS 29 can be switched.

The optical train of the image projection system can be configured withone or more of the WGP-PBSs 14 a-c or analyzers also forming a combinerto combine two or more colored beams. Referring to FIG. 2a, an exemplaryimage projection system 10 c is shown configured with the WGP-PBSs 14a-c configured as both analyzers and combiners, or to both 1) separatethe encoded image information, and 2) combine the various differentcolored beams. The image projection system 10 c is similar in manyrespects to those described herein, and can be configured for threedifferent colors with three transmissive arrays 16 a-c.

A first polarizing beam splitter or WGP-PBS 28 can be disposed in twocolored modulated beams from two transmissive arrays 16 a and b. Thefirst polarizing beam splitter or WGP-PBS 28 can both 1) separate theencoded image information in both of the modulated beams (separatingboth modulated beams into reflected and transmitted beams as describedabove), and 2) combing the reflected beam from one of the modulatedbeams, with the transmitted beam of the other modulated beam, into afirst combined beam with the desired image information for two of thecolored beams. It will be noted that one of the modulated beams includesthe desired image information in one polarization, while the othermodulated beam includes the desired image information in anotherpolarization, so that the desired image information can be combined intothe first combined beam by the first polarizing beam splitter or WGP-PBS28. For example, the desired image information from one modulated beamcan be in a polarization that is transmitted by the WGP-PBS 28, whilethe desired image information from the other modulated beam can be in apolarization that is reflected by the WGP-PBS 28. For example, theWGP-PBS 28 can be disposed in the modulated beams from of the blue andgreen colors, and can combine the image information transmitted bluebeam with the image information in the reflected green beam, as shown.The WGP-PBS 28 is oriented with respect to both modulated beams, or bothtransmissive arrays 16 a and b, so that the desired reflected andtransmitted beam align as the desired first combined beam.

It will be appreciated that the desired image information can be encodedon the modulated beams differently, or that the transmissive arrays canbe configured differently. For example, a first transmissive array 16 acan encode the information on the modulated beam to be transmitted bythe WGP-PBS 28, while the second array 16 b can encode the informationon the modulated beam to be reflected by the WGP-PBS 28. Thus, the twomodulated beams can have the image information encoded in oppositepolarizations, or the two transmissive arrays can encode the imageinformation in opposite polarizations.

A second polarizing beam splitter or WGP-PBS 29 can be disposed in thefirst combined beam from the first WGP-PBS 28, and another coloredmodulated beam from another transmissive array 16 c. The secondpolarizing beam splitter or WGP-PBS 29 can both 1) separate the encodedimage information from the other modulated beam, and 2) combing thereflected and/or transmitted beam from the other transmissive array withthe first combined beam from the first WGP-PBS 28 into a second combinedbeam with the desired image information for three of the colored beams.For example, the second WGP-PBS 29 can combine the image information inthe reflected red beam with the first combined beam (or reflected greenbeam and transmitted blue beam).

Therefore, two polarizing beam splitters or WGP-PBSs 28 and 29 can beused to analyze and combine three colored modulated beams. A singleWGP-PBS can be used to analyze and combine two colored modulated beams.The beams can be combined such that one beam is reflected from the beamsplitter in such a manner as to cause the reflected beam to be collinearand with the image information registered upon the other beam which istransmitted through the beam splitter, the combined beams forming amulticolor image beam.

As shown in FIG. 2a, the WGP-PBSs 28 and 29 can be orientedsubstantially parallel with one another. Such a configuration can allowthe two WGP-PBSs 28 and 29 to be disposed closer together, and thus canreduce the size of the system 10 c. Referring to FIG. 2b, the system 10d can be configured with the WGP-PBSs 28 and 29 oriented substantiallyorthogonal to one another. Such a configuration can allow better opticalqualities of the WGP-PBSs 28 and 29 to be combined.

While the above systems have been described with respect to using threecolored beams, it will be appreciated that the system can be configuredto utilize more or less colored beams. In addition, the above systemshave been described as using separate colored beams, or separate opticalpaths for each color. It will be appreciated that two or more colorbeams can share the same optical path. For example, referring to FIG.2c, a system 10 e is shown with two optical paths, and a single WGP-PBS28 acting as booth the analyzer and the combiner. Two colors, forexample green and red, can share an optical path, as shown. Analternator 24 b can be used to alternate between colors in the opticalpath. For example, the alternator 24 b can switch between the green andred color beams. The transmissive array 16 b also can be controlled toalternate control between the different colors, and thus can selectivelyalter different colors. The switching of colored beams by the alternator24 b and transmissive array 16 b can be rapid enough so as to beundetected by the unaided eye. The alternator 24 b, or alternator meansfor altering between colors, can be a color wheel or the like.

As described above, the pre-polarizers 26 a-c also can be wire gridpolarizers. Referring to FIGS. 3a-b and 4 a-c, image projection systems10 f-j are shown that are similar to those described above, but withwire grid polarizers 26 d-f as the pre-polarizers. The wire gridpolarizers 26 d-f can be oriented at an acute or obtuse angle (asopposed to a right angle) with respect to the respective coloredpolarized beams so that the reflected beam is directed away from thelight source or separator.

The wire grid polarizers can be oriented to other optical elements torecycle the light. Referring to FIG. 4c, an exemplary system 10 j isshown with a light recycling apparatus 13. The apparatus 13 can includea plurality of mirrors or the like to collect the discarded light beamsof discarded polarizations. The apparatus 13 can also direct thediscarded light beams back to the light source or separator. While theapparatus 13 is shown in FIG. 4c for a single optical path, it will beappreciated that the apparatus can be configured to collect thediscarded light beams from the various different optical paths. Inaddition, the system 10 j is shown with a single WGP-PBS 16 e as apre-polarizer, and a single WGP-PBS 28 as an analyzer. Such aconfiguration can be used with a single color light beam. Alternatively,an alternator 24 b or color wheel can be used, as described above, torapidly alternate between different colors of light.

As described above, the WGP-PBSs can be used as the pre-polarizers 26a-c, the polarizing beam splitters or analyzers 14 a-c, and thecombiner(s).

For adequate optical efficiency, the WGP-PBS should have highreflectivity (R_(S)) of the desired polarization from the light source20, and it should have high transmissivity (T_(P)) of the oppositepolarization from the liquid crystals array 26. The conversionefficiency is proportional to the product of these two, R_(S)T_(P), sodeficiency in one factor can be compensated to some extent byimprovement in the other.

Examples of wire grid polarizing beam splitters 14 of the presentinvention advantageously show the following characteristics whichdemonstrate the advantage of using a WGP-PBS of the present invention asthe pre-polarizer, the analyzer, and/or the combiner in display devicesfor the visible portion of the spectrum. Theoretical calculations offurther improvements indicate that even better polarizing beam splitterswill be available.

Referring to FIGS. 5a and 5 b, the measured transmissivity andreflectivity, respectively, for both S and P polarizations of a WGP-PBSare shown. In FIG. 5c, the efficiency of the WGP-PBS is shown as theproduct of the transmissivity and reflectivity. In addition, theextinction is also shown in FIG. 5c. In FIGS. 5a-c, the WGP-PBS isoriented to reflect the s-polarization and transmit the p-polarizationat incident angles of 30 degrees, 45 degrees and 60 degrees. For animage projection system, such as a projector, the product of thereflected s-polarization and transmitted p-polarization (R_(S)T_(P))must be large if source light is to be efficiently placed on the screen.On the other hand, for the resolution needed to achieve high informationdensity on the screen, it is important that the converse product(R_(P)T_(S)) be very small (i.e. the transmission of s-polarized lightmultiplied by the reflection of p-polarized light must be small). It isclear from the figures that the wire grid polarizing beam splitter ofthe present invention meets these standards over the entire spectrumwithout degradation by Rayleigh resonance or other phenomena.

Another important feature is a wide acceptance angle. This must be largeif light gathering from the source, and hence the conversion efficiency,is maximized. Referring to FIG. 6, the performance of the wire gridpolarizing beam splitter of the present invention is shown for variousportions of the light cone centered around the optical axis which isinclined at 45 degrees. In FIG. 6, the first referenced angle is theangle in the plane of incidence while the second referenced angle is theangle in the plane perpendicular to the plane of incidence. It is clearthat the WGP-PBS of the present invention is able to accept cones oflight (either diverging or converging) with half-angles betweenapproximately 12 and 25 degrees.

Referring to FIGS. 7a-c, theoretical calculations for an alternativeembodiment of a wire grid polarizing beam splitter indicate thatsignificantly larger light cones and/or other enhancements will bepossible. FIGS. 4a and 4 b show the theoretical throughput andextinction, respectively, of a wire grid polarizing beam splitter with aperiod p reduced to 130 nm. In addition, the grid height or thickness is130 nm; the line-spacing ratio is 0.48; the substrate groove depth is 50nm; and the substrate is BK7 glass. It should be noted in FIG. 7a thatthe throughput is grouped much more closely than the throughput shown inFIG. 5c. Therefore, performance can be improved by reducing the periodp. It should be noted in FIG. 7b that the extinction is significantlyincreased in comparison to FIG. 5c.

FIG. 7c shows the theoretical extinction of another alternativeembodiment of the wire grid polarizing beam splitter with the period pfurther reduced. The wavelength is 420 nm and the incidence angle is 30degrees. It should be noted that the extinction increases markedly asthe period p is reduced.

As indicated above, an important consequence of the ability to acceptlarger light cones with a WGP-PBS that will work well at large angles isthat the PBS no longer restricts the optical design of the imagingsystem. Thus, conventional light sources can be used, with the advantageof their low cost, cooler operation, small size, and low weight. Thewide range of angles over which the WGP-PBS works well makes it possiblefor the designer to position the other optical elements in favorablepositions to improve the size and operation of the display.

Referring to FIGS. 1-4, the design flexibility provided by the WGP-PBSsis demonstrated. In addition, the WGP-PBSs provide wide range of angles,also contributing to design flexibility.

Yet other features of wire grids provide advantages for display units.The conventional technology requires the use of a glass cube. This cubeimposes certain requirements and penalties on the system. Therequirements imposed include the need to deal with thermal loading ofthis large piece of glass, the need for high quality materials withoutstress birefringence, etc., which impose additional cost, and the extraweight and bulk of the cube itself. The WGP-PBS of the present inventionadvantageously is a divided or patterned thin film that does not occupymuch volume and does not weigh very much. It can even be integrated withor incorporated into other optical elements such as color filters, tofurther reduce part count, weight, and volume of the projection system.

The WGP-PBS of the present invention is also very robust. Modem lightsources generate very high thermal gradients in the polarizerimmediately after the light is switched on. At best, this can inducethermal and stress birefringence which causes cross talk betweenpolarizations. At worst, it can delaminate multilayer polarizers orcause the cemented interface in a cube beam splitter to separate. Whatis more, the long duration of exposure to intense light causes somematerials to change properties (typically yellowing fromphoto-oxidation). However, wire grid polarizing beam splitters are madeof chemically inert metal that is well attached to glass or othersubstrate materials. They have been shown to withstand high temperaturesas well as long periods of intense radiation from light sources.

The WGP-PBS of the present invention also is easy to align. It is asingle part that needs to be adjusted to direct the source beam onto theliquid crystal array. This is the same simple procedure that would beused for a flat mirror. There is another adjustment parameter, namely,the angular rotation about the normal to the WGP surface. Thisdetermines the orientation of polarization in the light beam. Thisadjustment is not critical because the WGP functions as its own analyzerand cannot be out of alignment in this sense. If there are otherpolarizing elements in the optical train, the WGP-PBS should be orientedwith respect to their polarization, but slight misalignment is notcritical because: according to Malus' law, angular variation makes verylittle difference in the intensity transmitted by polarizers if theirpolarization axes are close to being parallel (or perpendicular).

In order to be competitive with conventional polarizers, the productR_(S)T_(P) must be above approximately 50%. This represents a lowerestimate which would only be practical if the WGP-PBS was able to gathersignificantly more light from the light source than the conventionalpolarizing beam splitters. The estimate of 50% comes from an assumptionthat the best conventional beam splitter, a modern MacNeille cube beamsplitter, can deliver an f/# of about f/2.5 at best. An optical systemwhich was twice as fast, or capable of gathering twice as much light,would then have an f/# of ½ of this value, or about f/1.8, which iscertainly a reasonable f/# in optical image projection systems. A systemwhich is twice as fast, and therefore capable of gathering twice thelight from the source, would approximately compensate for the factor of2 decrease in the R_(S)T_(P) product over the conventional cube beamsplitter, resulting in an equivalent projection system performance. Infact, since a WGP-PBS can potentially be used down below f/1.2 (a factorof four increase) this seemingly low limit can still produce very brightimages. Of course, an R_(S)T_(P) product which is over this minimumvalue will provide even better performance.

Another important performance factor is contrast in the image, asdefined by the ratio of intensities of light to dark pixels. One of thesignificant advantages of the WGP-PBS is the improved contrast overcompound incident angles in comparison to the prior art cube beamsplitter such as a McNeille prism. The physics of the McNeille prismpolarizes light by taking advantage of the difference in reflectivity ofS vs. P polarization at certain angles. Because S and P polarization aredefined with respect to the plane of incidence, the effective S and Ppolarization for a particular ray in a cone of light rotates withrespect to the ray along the optical axis as various rays within thecone of light are considered. The consequence of this behavior is thewell-known compound angle problem in which the extinction of thepolarizer is significantly reduced for certain ranges of angles withinthe cone of light passing through the polarizing beam splitter,significantly reducing the average contrast over the cone.

The WGP-PBS, on the other hand, employs a different physical mechanismto accomplish the polarization of light which largely avoids thisproblem. This difference in behavior is due to the fact that thepolarization is caused by the wire grids in the beam splitter which havethe same orientation in space regardless of the plane of incidence forany particular ray in the cone of light. Therefore, even though theplane of incidence for any particular ray is the same when incident on aMcNeille prism or a WGP, the polarization effect is only dependent onthe plane of incidence in the case of the McNeille prism, meaning thecompound angle performance of the WGP is much improved over thatprovided by the cube beam splitter.

The fact that the function of the WGP-PBS is independent of the plane ofincidence means that the WGP-PBS can actually be used with the wires orelements oriented in any direction. The preferred embodiment of theinvention has the elements oriented parallel to the axis around whichthe polarizer is tilted so that the light strikes the WGP-PBS at anangle. This particular orientation is preferred because it causes thepolarization effects of the surface reflections from the substrate to beadditive to the polarization effects from the grid. It is possible,however, to produce a WGP-PBS which functions to reflect theP-polarization and transmit the S-polarization (which is exactlyopposite what has been generally described herein) over certain rangesof incident angles by rotating the grid elements so they areperpendicular to the tilt axis of the WGP-PBS. Similarly, the gridelements can be placed at an arbitrary angle to the tilt axis to obtaina WGP-PBS which functions to transmit and reflect light withpolarizations aligned with the projection of this arbitrary angle ontothe wavefront in the light beam. It is therefore clear that WGP-PBSwhich reflect the P-polarization and transmit the S-polarization, orwhich reflect and transmit light with polarization oriented at arbitraryangles are included within this invention.

The compound angle performance advantage of the WGP-PBS provides aninherently more uniform contrast over the entire light cone, and is oneof the reasons the WGP is suitable for very small f-numbers. But, ofcourse, it is not the only factor affecting the image contrast. Theimage contrast is governed to a large extent by low leakage of theundesired polarization, but in this case the product T_(S)R_(P) is notthe important parameter, because the image generating array which liesin sequence after the first encounter with the beam splitter but beforethe second also takes part in the production of the image contrast.Therefore, the final system contrast will depend on the light valveperformance as well as the polarizer extinction. However, lower boundson the required beam splitter performance can be determined with theassumption that the light valve performance is sufficient enough that itcan be assumed to have an essentially infinite contrast. In this case,the system contrast will depend entirely on the beam splitterperformance.

Referring to FIGS. 1-4c, there are several different functions fulfilledby the WGP-PBS, such as a pre-polarizer, analyzer or beam splitter,and/or combiner. In addition, some of the WGP-PBSs fulfill two differentfunctions, such as both the combiner and the analyzer or beam splitter.One function is the preparation of the polarized light before it strikesthe transmissive array 16 a-c or other suitable image generation device.The requirement here is that the light be sufficiently well polarizedthat any variations in the polarization of the light beam created by thelight valve can be adequately detected or analyzed such that the finalimage will meet the desired level of performance. Similarly, theanalyzer or beam splitter 14 a-c must have sufficient performance toanalyze light which is directed by the light valve or transmissive arrayto the beam splitter so that the desired system contrast performance isachieved. In addition, the combiner must have sufficient performance toboth transmit and reflect the light to combine the different colors.

These lower bounds can be determined fairly easily. For reasons ofutility and image quality, it is doubtful that an image with a contrastof less than 10:1 (bright pixel to adjacent dark pixel) would have muchutility. Such a display would not be useful for dense text, for example.If a minimum display system contrast of 10:1 is assumed, then anincident beam of light is required which has at least 10 times the lightof the desired polarization state over that of the undesiredpolarization state. In terms of polarizer performance, this would bedescribed as having an extinction of 10:1 or of simply 10.

As an analyzer, the beam splitter 14 must analyze the image, and must beable to pass the light of the right polarization state, whileeliminating most of the light of the undesired state. Again, assumingfrom above a light beam with an image encoded in the polarization state,and that this light beam has the 10:1 ratio assumed, then a beamsplitter is desired which preserves this 10:1 ratio to meet the goal ofa system contrast of 10:1. In other words, it is desired to reduce thelight of the undesired polarization by a factor of 10 over that of theright polarization. This again leads to a minimum extinction performanceof 10:1 for the analysis function of the beam splitter.

Clearly, higher system contrast will occur if either or both of thepolarizer and analyzer functions of the beam splitter have a higherextinction performance. It is also clear that it is not required thatthe performance in both the analyzer function and the polarizer functionof the beam splitter be matched for an image projection system toperform adequately. An upper bound on the polarizer and analyzerperformance of the beam splitter is more difficult to determine, but itis clear that extinctions in excess of approximately 20,000 are notneeded in this application. A good quality movie projection system asfound in a quality theater does not typically have an image contrastover about 1000, and it is doubtful that the human eye can reliablydiscriminate between an image with a contrast in the range of severalthousand and one with a contrast over 10,000. Given a need to produce animage with a contrast of several thousand, and assuming that the lightvalves capable of this feat exist, an upper bound on the beam splitterextinction in the range of 10,000-20,000 would be sufficient.

The above delineation of the minimum and maximum bounds on the wire gridbeam splitter is instructive, but as is clear from the demonstrated andtheoretical performance of a wire grid beam splitter as shown above,much better than this can be achieved. In accordance with thisinformation, the image projection system can have a R_(S)T_(P) of 65%,and R_(P) or T_(S) or both are 67%, as shown in FIGS. 5a-c.

A post-polarizer or clean-up polarizer can be disposed in the modulatedbeam, or between the WGP-PBS 14 and the screen 18. The image displaysystem may also utilize light gathering optics and projection optics.LCD compensation films 17 can be disposed on either side of thetransmissive array 16.

Referring to FIGS. 8 and 9, the wire grid polarizing beam splitter 14 ofthe present invention is shown in greater detail. The polarizing beamsplitter is further discussed in greater detail in U.S. Pat. No.6,243,199, issued Jun. 5, 2001, which is herein incorporated byreference.

The polarizing beam splitter 14 has a grid 30, or an array of parallel,conductive elements, disposed on a substrate 40. The source light beam130 produced by the light source 20 is incident on the polarizing beamsplitter 14 with the optical axis at an angle Θ from normal, with theplane of incidence preferably orthogonal to the conductive elements. Analternative embodiment would place the plane of incidence at an angle Θto the plane of conductive elements, with Θ approximately 45 degrees.Still another alternative embodiment would place the plane of incidenceparallel to the conductive elements. The polarizing beam splitter 14divides this beam 130 into a specularly reflected component 140, and atransmitted component 150. Using the standard definitions for S and Ppolarization, the light with S polarization has the polarization vectororthogonal to the plane of incidence, and thus parallel to theconductive elements. Conversely, light with P polarization has thepolarization vector parallel to the plane of incidence and thusorthogonal to the conductive elements.

Ideally, the polarizing beam splitter 14 will function as a perfectmirror for the S polarized light, and will be perfectly transparent forthe P polarized light. In practice, however, even the most reflectivemetals used as mirrors absorb some fraction of the incident light, andthus the WGP will reflect only 90% to 95%, and plain glass does nottransmit 100% of the incident light due to surface reflections.

The physical parameters of the wire grid beam splitter 14 which shouldbe optimized as a group in order to achieve the level of performancerequired include: the period p of the wire grid 30, the height orthickness t of the grid elements 30, the width w of the grids elements30, and the slope of the grid elements sides. It will be noted inexamining FIG. 9 that the general cross-section of the grid elements 30is trapezoidal or rectangular in nature. This general shape is also anecessary feature of the polarizing beam splitter 14 of the preferredembodiment, but allowance is made for the natural small variations dueto manufacturing processes, such as the rounding of corners 50, andfillets 54, at the base of the grid elements 30.

It should also be noted that the period p of the wire grid 30 must beregular in order to achieve the specular reflection performance requiredto meet the imaging fidelity requirements of the beam splitter 14. Whileit is obviously better to have the grid 30 completely regular anduniform, some applications may have relaxed requirements in which thisis not as critical. However, it is believed that a variation in period pof less than 10% across a meaningful dimension in the image (such as thesize of a single character in a textual display, or a few pixels in animage) is required to achieve the necessary performance.

Similarly, reasonable variations across the beam splitter 14 in theother parameters described, such as the width w of the grid elements 30,the grid element height t, the slopes of the sides, or even the cornerrounding 50, and the fillets 54, are also possible without materiallyaffecting the display performance, especially if the beam splitter 14 isnot at an image plane in the optical system, as will often be the case.These variations may even be visible in the finished beam splitter 14 asfringes, variations in transmission efficiency, reflection efficiency,color uniformity, etc. and still provide a useful part for specificapplications in the projection imaging system.

The design goal which must be met by the optimization of theseparameters is to produce the best efficiency or throughput possible,while meeting the contrast requirements of the application. As statedabove, the minimum practical extinction required of the polarizing beamsplitter 14 is on the order of 10. It has been found that the minimumrequired throughput (R_(S)T_(P)) of the beam splitter 14 in order tohave a valuable product is approximately 50%, which means either or bothof R_(P) and T_(S) must be above about 67%. Of course, higherperformance in both the throughput and the extinction of the beamsplitter will be of value and provide a better product. In order tounderstand how these parameters affect the performance of the wire gridbeam splitter, it is necessary to examine the variation in performanceproduced by each parameter for an incident angle of 45 degrees, andprobably other angles of interest.

The performance of the wire grid beam splitter 14 is a function of theperiod p. The period p of the wire grid elements 30 must fall underapproximately 0.21 μm to produce a beam splitter 14 which has reasonableperformance throughout the visible spectrum, though it would be obviousto those skilled in the art that a larger period beam splitter would beuseful in systems which are expected to display less than the fullvisible spectrum, such as just red, red and green, etc.

The performance of the wire grid beam splitter 14 is a function of theelement height or thickness t. The wire-grid height t must be betweenabout 0.04 and 0.5 μm in order to provide the required performance.

The performance of the wire grid beam splitter 14 is a function of thewidth to period ratio (w/p) of the elements 30. The width w of the gridelement 30 with respect to the period p must fall within the ranges ofapproximately 0.3 to 0.76 in order to provide the required performance.

The performance of the wire grid beam splitter 14 is a function of theslopes of the sides of the elements 30. The slopes of the sides of thegrid elements 30 preferably are greater than 68 degrees from horizontalin order to provide the required performance.

It is to be understood that the above-referenced arrangements areillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements can bedevised without departing from the spirit and scope of the presentinvention while the present invention has been shown in the drawings anddescribed above in connection with the exemplary embodiments(s) of theinvention. It will be apparent to those of ordinary skill in the artthat numerous modifications can be made without departing from theprinciples and concepts of the invention as set forth in the claims.

What is claimed is:
 1. An image projection system, for providing abright and clear image, the system comprising: a) a light source capableof producing a visible light beam; b) at least one first polarizer,located near the light source in the visible light beam, configured topolarize the light beam into a polarized light beam; c) at least onetransmissive array, located near the first polarizer in the polarizedlight beam, capable of modulating the polarization of the polarizedlight beam by selectively altering the polarization of the polarizedlight beam to encode image information thereon and creating a modulatedbeam; d) at least one polarizing beam splitter, located near thetransmissive array in the modulated beam, and including: 1) atransparent substrate having a first surface located in the modulatedbeam, and 2) a generally parallel arrangement of thin, elongatedelements, supported by the substrate, the arrangement being configuredand the elements being sized to interact with electromagnetic waves ofthe modulated beam to generally (i) transmit light through the elementswhich has a polarization oriented perpendicular to a plane that includesat least one of the elements and the direction of the incident lightbeam, defining a transmitted beam, and (ii) reflect light from theelements which has a polarization oriented parallel with the plane thatincludes at least one of the elements and the direction of the incidentlight beam, defining a reflected beam; and e) the polarizing beamsplitter being oriented at an angle with respect to the modulated beamsuch that the modulated beam strikes the first surface of thetransparent substrate at an angle and the reflected beam is directedaway from the transmissive array; and f) a screen, located in either thereflected beam or the transmitted beam, for displaying the encoded imageinformation.
 2. A system in accordance with claim 1, wherein: a) thefirst polarizer further includes a first polarizing beam splitter,located in the light beam, the first beam splitter comprising: 1) atransparent substrate having a first surface located in the light beam,and 2) a generally parallel arrangement of thin, elongated elementssupported by the substrate, the arrangement being configured and theelements being sized to interact with electromagnetic waves of the lightbeam to generally (i) transmit light through the elements which has apolarization oriented perpendicular to a plane that includes at leastone of the elements and the direction of the incident light beam,defining a first polarized transmitted beam, and (ii) reflect light fromthe elements which has a polarization oriented parallel with the planethat includes at least one of the elements and the direction of theincident light beam, defining a first polarized reflected beam; and b)the transmissive array being located in either the first polarizedreflected beam or the first polarized transmitted beam.
 3. A system inaccordance with claim 1, wherein the polarizing beam splitter isoriented with respect to the modulated beam at an incident angle betweenapproximately 0 to 80 degrees.
 4. A system in accordance with claim 1,wherein the polarizing beam splitter is oriented with respect to themodulated beam at incidence angles greater than 47 degrees or less than43 degrees.
 5. A system in accordance with claim 1, wherein the lightbeam has a useful divergent cone with a half angle between approximately12 and 25 degrees.
 6. A system in accordance with claim 1, wherein thepolarizing beam splitter is used at an F-number less than approximatelyf/2.5.
 7. A system in accordance with claim 1, wherein the polarizingbeam splitter has a throughput of at least 50% defined by the product ofthe fractional amount of p-polarization transmitted light and thefractional amount of s-polarization reflected light; and wherein thes-polarization transmitted light and p-polarization reflected light areboth less than 5%.
 8. A system in accordance with claim 1, wherein thepolarizing beam splitter has a throughput of at least 50% defined by theproduct of the fractional amount of s-polarization transmitted light andthe fractional amount of p-polarization reflected light; and wherein thep-polarization transmitted light and s-polarization reflected light areboth less than 5%.
 9. A system in accordance with claim 1, wherein thepolarizing beam splitter has a throughput for the light beam of at least65%, defined by the product of the fractional amount of reflected lightand the fractional amount of transmitted light; and wherein the percentof reflected light or the percent of transmitted light is greater thanapproximately 67%.
 10. A system in accordance with claim 1, wherein: thearrangement of elements has a period less than approximately 0.21microns, the elements have a thickness between approximately 0.04 to 0.5microns, and the elements have a width of between approximately 30 to76% of the period.
 11. A device in accordance with claim 1, wherein thepolarizing beam splitter further includes: a) a second layer, separatefrom the transparent substrate, having a refractive index, the elementsbeing disposed between the transparent substrate and the second layer;and b) a plurality of gaps, formed between the elements, and having acontent providing a refractive index less than a refractive index of thetransparent substrate.
 12. A device in accordance with claim 11, whereinthe content of the gaps between the elements include air.
 13. A systemin accordance with claim 11, wherein the content of the gaps between theelements have a vacuum.
 14. A system in accordance with claim 11,wherein the content of the gaps between the elements include a materialdifferent from materials of the transparent substrate and the secondlayer.
 15. A system in accordance with claim 11, wherein the content ofthe gaps include a material that is a same material as the second layer.16. A system in accordance with claim 11, wherein the content of thegaps include a material that is a same material as the first transparentsubstrate.
 17. A system in accordance with claim 11, wherein the contentof the gaps between the elements include water.
 18. A system inaccordance with claim 11, wherein the content of the gaps between theelements include magnesium fluoride.
 19. A system in accordance withclaim 11, wherein the content of the gaps between the elements includeoil.
 20. A system in accordance with claim 11, wherein the content ofthe gaps between the elements include hydrocarbon compounds.
 21. Asystem in accordance with claim 11, wherein the content of the gapsbetween the elements include plastic.
 22. A system in accordance withclaim 11, wherein the content of the gaps between the elements includefloranated hydrocarbon.
 23. A system in accordance with claim 1, whereinthe arrangement has a configuration and the elements have a size whichwould normally create a resonance effect in combination with one of thelayers within the visible spectrum; and wherein content of the gaps witha lower refractive index than the refractive index of one of the layerscauses a shift of the normally occurring resonance effect to a lowerwavelength, thereby broadening a band of visible wavelengths in which noresonance effect occurs.
 24. An image projection system comprising: a) alight source capable of producing a visible light beam; b) a bandwidthseparator, located near the light source in the visible light beam, toseparate the visible light beam into a plurality of colored beams ofdifferent bandwidths; c) at least one first polarizer, located near thelight source in the visible light beam, configured to polarize the lightbeam; d) the separator and the at least one first polarizer togetherproducing a plurality of colored polarized beams; e) a plurality oftransmissive arrays, each disposed in one of the colored polarizedbeams, each capable of modulating polarization of the respective coloredpolarized beam by selectively altering the polarization of the coloredpolarized beam to encode image information thereon and creating acolored modulated beam; f) at least one polarizing beam splitter,located in at least two of the colored modulated beams, to both 1)separate the image information encoded on the at least two coloredmodulated beams, and 2) combine the at least two colored modulated beamsinto a combined beam, the at least one polarizing beam splitterincluding: 1) a transparent substrate having a first surface located inthe at least two colored modulated beams, and 2) a generally parallelarrangement of thin, elongated elements, supported by the substrate, thearrangement being configured and the elements being sized to interactwith electromagnetic waves of the colored modulated beams to generally(i) transmit light through the elements which has a polarizationoriented perpendicular to a plane that includes at least one of theelements and the direction of the incident light beam, defining acolored transmitted beam, and (ii) reflect light from the elements whichhas a polarization oriented parallel with the plane that includes atleast one of the elements and the direction of the incident light beam,defining a colored reflected beam; and g) a screen, located in thecombined beam, for displaying the encoded image information.
 25. Adevice in accordance with claim 24, wherein the at least one polarizingbeam splitter is oriented at an angle with respect to the at least twocolored modulated beams such that each of the colored modulated beamsstrikes the first surface of the transparent substrate of the at leastone polarizing beam splitter at an angle and the colored reflected beambeing directed away from the respective transmissive array.
 26. A devicein accordance with claim 24, wherein the at least one first polarizerincludes: a) a first transparent substrate having a first surfacelocated in the light beam, and b) a generally parallel arrangement ofthin, elongated elements supported by the substrate, the arrangementbeing configured and the elements being sized to interact withelectromagnetic waves of the light beam to generally (i) transmit lightthrough the elements which has a polarization oriented perpendicular toa plane that includes at least one of the elements and the direction ofthe incident light beam, defining a first transmitted beam, and (ii)reflect light from the elements which has a polarization orientedparallel with the plane that includes at least one of the elements andthe direction of the incident light beam, defining a first reflectedbeam.
 27. A device in accordance with claim 26, wherein the at least onefirst polarizer is oriented at an acute angle with respect to the lightbeam such that the light beam strikes the first surface of the firstpolarizing beam splitter at an acute angle.
 28. An image projectionsystem comprising: a) a light source capable of producing a visiblelight beam; b) a bandwidth separator, located near the light source inthe visible light beam, to separate the visible light beam into aplurality of colored beams each characterized by a different color withdifferent bandwidths; c) at least one first polarizer, located near thelight source in the visible light beam, configured to polarize the lightbeam; d) the separator and the at least one first polarizer togetherproducing a plurality of colored polarized beams; e) three transmissivearrays, each in one of the colored polarized beams, each capable ofmodulating polarization of the respective colored polarized beam byselectively altering the polarization of the colored polarized beam toencode image information thereon and creating a colored modulated beam;f) a first polarizing beam splitter, located near two transmissivearrays and in two different colored modulated beams, combining the twodifferent colored modulated beams into a first combined beam; g) asecond polarizing beam splitter, located near another transmissive arrayand the first polarizing beam splitter, combining a colored modulatedbeam from the another transmissive array and the first combined beaminto a second combined beam; h) each of the first and second polarizingbeam splitters including: 1) a transparent substrate having a firstsurface; and 2) a generally parallel arrangement of thin, elongatedelements, supported by the substrate, the arrangement being configuredand the elements being sized to interact with electromagnetic waves ofthe colored modulated beams to generally (i) transmit light through theelements which has a polarization oriented perpendicular to a plane thatincludes at least one of the elements and the direction of the incidentlight beam, and (ii) reflect light from the elements which has apolarization oriented parallel with the plane that includes at least oneof the elements and the direction of the incident light beam; and i) ascreen, located in the second combined beam, for displaying the encodedimage information.
 29. A system in accordance with claim 28, wherein thefirst and second polarizing beam splitters are oriented at an angle withrespect to the respective colored modulated beams such that each of thecolored modulated beams strikes the first surface of the transparentsubstrate of the respective polarizing beam splitter at an angle and arespective colored reflected beam being directed away from therespective transmissive array.